Cyanine dyes have been described in the literature for many years.sup.1,2, mainly for photographic purposes. In recent years, researchers have taken advantage of the excellent fluorescent properties of the carbocyanines to label biological molecules. Initial efforts were thwarted by the high background and/or quenching of fluorescence observed when the dyes were conjugated to proteins. The hydrophobic nature of the dyes caused them to aggregate in aqueous media or on the hydrophobic domains of proteins. Thus, the dyes, as described in the early literature, were not suitable for labeling.
Waggoner, et al..sup.3,4 disclosed the use of sulfonated derivatives of carbocyanines to label biological molecules. The sulfonate group was found to be effective at preventing aggregation, because of the repulsion of the negative charges between molecules. In some of the cited Waggoner disclosures, the importance of the sulfonate groups to the novelty and efficacy of the dye derivatives, which included nucleic acids, was emphasized.
U.S. Pat. No. 5,556,959 discloses the use of carbocyanine phosphoramidites to label synthetic oligonucleotides. Due to the constraints of the automated systems used for DNA synthesis, the amidites had to be soluble in aprotic organic solvents. Sulfonated carbocyanines are insoluble in the solvents best suited for oligonucleotide synthesis. Therefore, the dye amidites described in U.S. Pat. No. 5,556,959 lacked the sulfonate groups. Experiments showed that the amidites were soluble in the appropriate solvents, such as acetonitrile and dichloromethane, and labeled the oligonucleotides in high yield. The dye amidites and intermediates are easily and efficiently synthesized and purified.
Nucleoside triphosphates (NTPs) labeled with reporter groups have been in use for many years.sup.5,6. NTPs labeled with sulfonated carbocyanines have been reported in the scientific literature.sup.7a, and are commercially available.sup.7b. However, synthesis of sulfonated cyanines is a difficult procedure, and the purity of the dye intermediates used in labeling is variable. The recommended shelf-life is short. Reagents for labeling are therefore expensive, as are the labeled NTPs derived from them.
Needed in the art of molecular biology is a nonsulfonated cyanine dye attached to a nucleotide or nucleoside.